Add ctkCollapsibleButton::flat property

Closes #502
Merge branch '502-flat-ctkCollapsibleButton'

* 502-flat-ctkCollapsibleButton:
  Add ctkCollapsibleButton::flat property
ENH: Clear box icon replaced with a less harsh one, a simple 'x' that does not indicate error
Merge pull request #503 from cpinter/501-search-box-clear-icon

Clear box icon replaced with a less harsh one, a simple 'x' that do...
Merge branch 'master' into CTK-CX-modifications
Fixed compile issue in commandlinemodules: included Qt5::Widgets in order to use QApplication